CHEMICAL ENGINEERING 3 then jigged principally for the object of separating class (2), flat coal ices from classes (3) and (5), bone and pure slate with coal faces. The 3 mode of operating over the old way by straight jigging yield an increased u£u. up to nearly 20 per cent. 1 Anthracite Separator.—Figure 4 stows the centrifugal spiral separator for anthracite coal. In this design there are three receiving divisions or chutes at the top. By centrifugal force and greater ease in sliding the clean coal rides to the upper edge of the inner spirals and drops into the large outer spirals, being discharged at the bottom. The inner or slate spirals can be arranged so as to give two products. The surface of the slate spirals are arranged with deflecting elements to assist in the separation. A capacity from 5 to 10 tons an hour is claimed for these devices. Separation by Decrepitation Followed by Screening.—Minerals like calcite, fluorite and barite which decrepitate on heating may be separated from gangue by roasting folio wed by screening. Decortication and Separation of Fibers.—By decortication is meant the removal of a coat and since in the preparation of vegetable fibers the cortex must be removed at some stage of the operation, this term has taken on the additional definition of mechanically cleaning fibers. The cortex of flax is its valuable part and consists of a layer of fibers surrounding a woody core or center. Ramie consists of bundles of fibers held together by a very sticky gum and enclosed by cortex. In preparing flax it is passed through a series of corrugated or toothed rolls, the corrugations or teeth meshing more or less with one another. This breaks up the woody core without materially injuring the fibers and upon passing the flax through a threshing or scutching machine the woody core is removed. Preparatory to pasage through the breaking rolls the flax undergoes a fermentation process to open up the bundles of fibers composing an outer layer of the stems. The cortex of ramie can, though without much success, be removed by passing it through a beating engine with revolving beater arms. The ramie comes under the influence of the beaters as it passes over a fixed shoe. The action of the machine loosens some of the sticky binding gum but not all of it. Ramie while it is a very strong fiber, the strongest there is in tension, tends to break if bent sharply and the FIG. 4.—Centrifugal separator for anthracite.